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Ballast Stones?

(Before continuing, just wanmted to say that the shark post was a mistake!)

A few more notes on the use of stones as ballast in "underwater flyers." 

Stones have been found in the stomachs of penguins and otariids (the eared
seals: sea lions and fur seals) and is implied in fossil remains of
plesiosaurs (so I understand), but they are not found in cetaceans,
phocids (non-eared seals), ichthyosaurs or mosasaurs. Mike Taylor noticed
that the difference between the stone swallowers and the non-stone
swallowers was that the former used their pectoral "flippers" to attain
underwater "flight" through the water whereas the others use their tails
to swim through the water. The stone swallowers needed to do so because
they would,when moving slowly or remaining stationary, move upwards in the
water, towards the surface. The animals which travelled with tails didn't
have this problem, because they balance their movement with their pectoral
flippers (presumedly).

However, this still doesn't make sense to me. First of all, when stone
swallowers are swimming slowly or are stationary, they are usually at the
water surface - case in point otariids. When otariids swim under water
they are always moving fast and are very agile. As for penguins, they
simply don't loll around the water at all (AFAIK) - they are always
swimming super fast. I read in R. McNiell Alexander's book on biomechanics
(damn can't remember the title - it was a Scientific American book and was
something like How Animals Move) that penguins are about the same density
of water, and I assume this is without "ballast" stones.  And even if
otariids are more bouyant than water (thanks to all their blubber), the
same can be said about phocids. Again, I don't think it matters, when
either of the animals are moving fast, whether they have tail propulsion
or flipper propulsion.

I hope you're following me here (if you're not, it's my fault!). Basically
I'm questioning the idea of swallowed stones being used for ballast.

As an aside, uninterrupted underwater "flight" is achieved, at least in
penguins (not sure about plesiosaurs or otariids), by the upward and
downward strokes of its wings both creating thrust: on the down stroke the
fin is on a positive angle creating an upwards direction and forward
thrust. Then, on the upwards stroke, the wing is put in a negative angle,
so that it moves the penguin downward but still maintains a forward

I wonder if plesiosaurs managed their flippers the same way. Actually,
first I want to know if otariids do this ...

I'd love to hear any comments or additional info.